Light source



March 20, 1962 J. D. H. HUGHES 3,026,436

LIGHT SOURCE Filed March 10, 1959 2 Sheets-Sheet 1 INVENTOR JOHN DUNCANHORSFALL HUGHES BY Oz ws74;

March 20, 1962 J. D. H. HUGHES LIGHT SOURCE 2 Sheets-Sheet 2 Filed March10, 1959 ZZ 2Z M AVW INVENTOR JOHN DIEICAl-F HCHSFI-LLL HUGI"ES3,026,436 Patented Mar. 20, 1962 3,026,436 LIGHT SOURCE John DuncanHorsfall Hughes, Wantage, England, as-

signor to United Kingdom Atomic Energy Authority, London, England FiledMar. 10, 1959, Ser. No. 798,379 Claims priority, application GreatBritain Mar. 12, 1958 13 Claims. (Cl. 31354) The invention relates tolight sources in which the source of energy producing the light is aradioactive gas or vapour emitting beta radiation.

One such known source comprises a transparent bulb coated internallywith a thin layer of phosphorescent material and filled with radioactivegas. The layer of phosphorescent material is thus activated by radiationon one of its surfaces and light is emitted from the other of itssurfaces. The bulb may be placed at the focus of a parabolic reflector.

According to the present invention, a light source comprises a gas-tightcontainer, a beta-radioactive gas within said container, and a coatingof phosphorescent material on a portion of the internal surface of saidcontainer, the container being light-transparent at least over a portionof the remainder of said surface. The invention enables thephosphorescent coating to be of optimum thickness irrespective of itsopacity to light, that is, to be of a thickness to absorb substantiallythe whole of the beta-radiation from the radioactive gas which isincident thereon. Most of the light emitted from the phosphorescentmaterial is thus emitted from the surface of the coating on which thebeta-radiation is incident. Emission of substantially all the light fromthis surface may be ensured by inter-posing a layer of light-reflectingmaterial between the phosphorescent material and the surface of thecontainer.

The radioactive gas may comprise any radioactive isotope emitting betaradiation, but the isotope is preferably one which emits very little, ifany, gamma radiation, is chemically non-toxic, and has a reasonably longhalflife. The preferred isotope is krypton-85, which has only a lowintensity gamma radiation associated with its beta radiation, ischemically non-toxic, being an inert gas, and has ahalf-life of aboutyears.

The phosphorescent material for coating the internal surface of thecontainer may be, for example, zinc sulphide, cadmium sulphide, zincphosphate or zinc silicate, activated by small amounts of other metalcompounds as is well known in the m. The required thickness of the layerof phosphorescent material to absorb substantially all the betaradiation of krypton-85 is about 2 mm. A suitable thickness to absorb amajor part of the beta radiation is 1 mm. A suitable light-reflectingmaterial for interposition between the layer of phosphorescent materialand the internal surface of the container is a dense white pigment suchas titanium dioxide.

In one particular advantageous form of the invention, the portion of theinternal surface of the container which is coated with thephosphorescent material is made a major portion of the surface, thecontainer being lighttransparent over substantially all the remainder ofthe surface. Thus the light emitted from the phosphorescent material isefiectively collimated through the transparent portion and the apparentbrightness of the light source is greater than the intrinsic brightnessof the surface of the phosphorescent material. In this form, thecontainer for the radioactive gas consists of a body with a cavity in ithaving an aperture, the internal surface of the cavity being coated witha layer of the phosphorescent material, and the aperture being closed bya transparent cover. The cover preferably has a plane internal surfaceto confine the radioactive gas to the cavity. The area of the apertureis less than the area of the coating of phosphorescent material and itmay with advantage be less than half, or even less than a quarter, ofthe area of the coating in order to obtain greater intensity of lighttransmitted through the aperture.

A proportion of a quarter, or less, can be achieved for example bymaking the cavity rectangular with a square aperture, part-sphericalwith a circular aperture, cylindrical with the axis of the cylinderperpendicular to the plane of a circular aperture, cylindrical with ahemispherical bottom and a circular aperture, or rectangular with asemi-cylindrical bottom and a square aperture, provided that the maximumdepth of the cavity is made at least equal to the side of the squareaperture or the diameter of the circular aperture.

A proportion of half, or less, is achieved by making the cavityrectangular, or rectangular with semi-cylindrical ends, provided thatthe maximum depth of the cavity is made at least equal to half the widthof the aperture.

A proportion of or less, is achieved by making the cavitypart-cylindrical with the axis of the cylinder parallel to the plane ofthe aperture, or part-cylindrical with part-spherical ends, orrectangular with a semi-cylindrical bottom, or rectangular withsemi-cylindrical ends and a semi-cylindrical bottom withquarter-spherical ends, provided that the maximum depth of the cavity ismade at least equal to half the width of the aperture.

The cavity may be formed in a body of any suitable material, such as ametal or a plastic, which is impermeable and chemically resistant to theradioactive gas or vapour. A suitable plastic is polymethylmethacrylate.

The material sunrounding the cavity may itself be made thick enough toreduce the transmission of any gamma radiation through it to a safe, lowlevel, or further shielding, for example lead, may surround it.

The aperture may be closed by a cover of any suitable gas-impermeablematerial, transparent to visible radiation, such as glass or transparentplastic, e.g. polymethyl methacrylate, but if the glass or plastic isadversely affected by beta radiation from the radioactive gas, anintermediate layer of transparent radiationuesistant material, forexample ceria-stabilised glass, may be interposed between it and thegas. The glass or plastic which forms the cover may also comprise aphosphorescent material, which will enhance the brightness of the lightsource. For example, a phosphor glass may be used.

The radioactive gas or vapour may be introduced into the cavity by anysuitable means. Preferably it is introduced into the cavity, afterclosure of the aperture, by means of a capillary which is sealedsubsequently.

The nature of the invention and the manner in which it is to beperformed will be made more apparent by the following description of twoparticular embodiments of the invention, by way of example, which areillustrated in the accompanying drawings, in which:

FIG. 1 is a section taken on a diameter of a circular light source;

FIG. 2 is a section on the line IlII of FIG. 1

FIG. 3 is a longitudinal section through another form of the invention;

FIG. 4 is a section on the line IVIV of FIG. 3, which is itself asection on the line IIIIII of FIG. 4; and

FIG. 5 is a partially cut-away view of the embodiment shown in sectionin FIGS. 3 and 4.

Referring first to the embodiment shown in FIGS. 1 and 2, the lightsource comprises a polymethyl methacrylate cup 1, defining a cavity 2 inthe form of a calibration standards.

hemisphere 2a with a short cylindrical part 212 surmounting thehemisphere. The cup 1 is coated on its inner surface with a layer ofphosphor 3 about 1 mm. thick. The cup is about 2 thick, has an internaldiameter of 2.2 cm., and is provided with a capillary 4 whichcommunicates initially with the outside of the cup. The cup is closed bya plane polymethyl methacrylate cover 5 to which is cemented aprotective disc of ceriastab ili'sed glass 6 fitting within the apertureof the cup The closed cup is surrounded on all sides, except that of theaperture, by a lead shield 7, provided with a hole 8 to accommodate thecapillary 4.

The cavity 2 is connected byrneans of the capillary 4, first to anexhaust pump to reduce the pressure within the cavity to a low value,and then to a source of krypton gas containing krypton-85 wherebyradioactive 'gas is admitted to the cavity. The capillary 4 is then heatsealed at 9 to retain the gas within the cavity.

In a light source constructed as in this particular embodiment, the areaof the cavity which is coated with phosphor is about 9 sq. cm., and thearea of the aperture is @314 sq. cm; thus the area of the aperture'isapproximately a third of the area of the phosphor coating. Thisproportion may be further decreased, and the brightness of the lightsource further increased, by increasing the length of the cylindricalpart 2b of the cavity. a

Referring now to the embodiment shown in FIGS. 3, 4 and 5, aself-luminous direction sign comprises a polymethyl methacrylate sheet21 in which are cut cavities consisting of channels having asemi-cylindrical cross-section of radius'S mm, forming the letters 22a,b, c and d. The cavities forming the letters are each coated with al'ayer of phosphor, 23a, b, c,'d about 1 mm. thick. The sheet 21 iscemented aroundits slightly raised periphcry to a second sheet ofpolymethyl methacrylate 25, leaving a gap between the central parts ofthe sheets 21 and 25 to provide a free gas path between the cavitiesforming the individual letters. One of the cavities forming the letter22a, is provided with a capillary 4 which communicates with a socket 26in the opposite face of the sheet 21.

The cavities forming the letters 22a, 12, c, d are connected by means ofthe capillary 24, first to an exhaust pump, connected by suitable meansto the socket 26, to reduce the pressure in the cavities to a low value,and then to a source of krypton gas containing krypton-85, wherebyradioactive gas is admitted to the gap between the sheets 21 and 25 andto the cavities. A plug is then fixed in the socket 26 to retain the gaswithin the cavities forming the letters 22a, b, c, d.

In the direction sign constructed according to this particularembodiment, the proportion of the area of the aperture for each of theletters to the area of the cavity proportion is This proportion could bedecreased by forming the cavities as channels with a U-shapedcross-section, i.e., by increasing the depth of the cavities.

Light sources using a radioactive gas or vapour as their source ofenergy have numerous'applications where electric power is not'availableor is undesirable, where electric power failure may be embarassing, orwhere long power cables are a disadvantage. For example, such lightsources may be used as personnel, package or safety markers in mines,stores, dark rooms, etc.;' on door handles and switches for location inthe dark; as signalling lamps; on large clock faces; or as photometricThelight sources of the present invention have particu- 4 lar advantagesin that the whole of the visible radiation emitted from the phosphor istransmitted as a beam from a small aperture with some directionalproperties. "the directional properties of the beam may be furtherimproved by fitting a lens over the aperture. The intensity of lightsources made in accordance with the invent on 13 greater than that ofpreviously known sources using a radioactive gas as energy source.Intensities of up to 30 111K (3000 micro-lamberts) have been achieved inlight sources constructed as the specific embodiment illustrated inFIGS. 1 and 2, and containing 0.8 curie of krypton-85. Sources of suchintensity are visible at distances of 500 to 1000 yards in darkness ortwilight, when fitted with a lens over the aperture.

Signs and lettering embodying the invention, constructed as the specificembodiment illustrated in FIGS. 3, 4 and 5 have particular value asdirection indicators, warning signs, and the like.

Light sources constructed in manner similar to either of the embodimentsmay be further modified by the incorporation of opaque signs, letteringor numerals m the transparent cover for the aperture, the l1ght sourcethus forming a luminous background for the signs,.letters, etc, andthrowing them into sharp relief.

I claim:

1. A light source comprising a gas-tight container, at beta-radioactivegas within said container, and a coat1ng of phosphorescent material on aportion of the internal surface of said container, the container beinglighttransparent at least over a portion -ofthe remainder of saidsurface. e

2. A light source as claimed in claim 1, in wh1ch sa1d coating is of athickness sufficient to absorb substantially all the beta-radiation fromsaid gas incident on said coating.

3. A light source as claimed in claim 1, in which a layer oflight-reflecting material is interposed between said coating and saidsurface. l p

4. A light source as claimed in claim 1, in which the portion of theinternal surface of the container which 15 coated with thephosphorescent material is a majorportion of said surface and thecontainer is light-transparent over substantially all the remainder ofsaid surface.

5. A light source as claimed in claim 4, in which said containercomprises a body defining a recess cavity in its surface and a lighttransparent cover closing the aperture of said cavity, the walls of saidcavity being coated with said phosphorescent material.

6. A light source as claimed in claim 5, in which said cavity is ahemispherical cavity.

7. A light source as claimed in claim 5, in which said cavity ispart-spherical with a circular aperture, the maximum depth of saidcavity being at least equal to the diameter of said aperture.

8. A light source as claimed in claim 5 in which said light-transparentcover has a plane internal surface.

9. A light source as. claimed in claim 5, in which said cavity is in theform of a channel of semicircular crosssection with closed ends.

10. A light source as claimed in claim 9 in which said channel is shapedto form readable indicia of a sign.

11. A light source as claimed in claim 5, in which said cavity is in theform of a hemisphere surmounted by a right circular cylinder of diameterequal to that of a said hemisphere.

References Cited in the file of this patent UNITED STATES PATENTSGoldstein Mar. 1, 1945 Linder Feb. 16', 1954

